Description

When importing a work with an author and co-author, neither of which has an Archive account, the byline on the work only shows one author.

This appears to be due to line 116 in application_helper.rb which reassigns the contents of the archivists entry for the current archivist instead of appending to it. Line 123 will also need to be amended to handle multiple external authors associated with the same archivist.

This case also requires a corresponding automated test to check the byline behaviour when there is more than one external author.

Testing:
This also removes duplication in the byline code.

check that existing stories have the same byline they normally should have (including Anonymous)

check that importing a work as an archivist with an author and co-author who do not have accounts on the Archive displays both authors in the format name1 [archived by archivist], name2 [archived by archivist]